Tracking control



July 25, 1950 E. B. FERRELL 2,516,765

TRACKING coN'rRoL Filed Feb. 24, 1944 9 Sheets-Sheet 1 /Nv/vro@ EB FERRELL ATroRA/Ey @V75/@MM July 25, 195o E. B. FERRI-:LL 2,516,755

TRACKING CONTROL Filed Feb. 24, 1944 9 Sheets-Sheet 2 RETIMCT/LE SPR/NG A-C OUTPUT A-C OUTPUT HHHHHHH 72 .4-c ourPur /Nvg/vrof? EB. FERREL L BK' A TTOR/VEV July 25, 1950 E. B. FERRELL. 2,516,765

TRACKING coN'rRoL Filed Feb. 24, 1944 9 sheets-sheet s /NVEN Tof? EBFERRE/.L

A 7' TOR/VEV July 25, 1950 E. B. FERRELL 2,516,765

TRACKING coN'rRQL Filed Feb. 24, 1.944 9 Sheets-Sheet 4 cENTRlFus/L c1. UTGHES July 25, 1950 E. B. FERRELL 2,516,765

TRACKING CONTROL Filed Feb. 24, 1944 9 Sheets-Sheet 5 A TTOR/VEV July 25, 1950 u. B. FERRELL 2,516,765

TRAGKING coNTRoL Filed Feb. 24, 1944 9 Sheets-Sheet 6 ATTORNEY July 25, 1950 E. B. FERRI-:LL

TRACKING coNTRoL FOR/VE V July 25, 1950 E. B. FERRELL TRACKING coNTRoL 9 Sheets-Sheet 8 Filed Feb. 24, 1944 ATTORNEY Patented July 25, 1950 TRACKING CONTROL Enoch B. Ferrell, Chatham, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 24, 1944, Serial No. 523,717

` This invention relates to tracking apparatus and more particularly to apparatus for continuously locating or indicating the position of a distant object or target which may be moving at a varying rate.

An object of the invention is to provide im-` proved electrical apparatus for controlling the angular velocity and phase of a controlled or output shaft in response to the angular displacement of an input or controlling shaft.

Another object of the invention is to provide apparatus for tracking an object automatically under control of signaling energy supplied thereto and a manually controllable apparatus for tracking the object which is automatically made effective for tracking control upon failure of the automatic tracking apparatus to perform the tracking function.

Another object is to provide apparatus for tracking an object automatically under control of signaling energy supplied thereto, a manually controllable apparatus for tracking the object, and means for causing the automatic tracking apparatus to control the manually controllable apparatus during periods when the automatic apparatus is performing the tracking function to condition the manually controllable apparatus ,forperforming the tracking function upon cessation of the automatic apparatus to perform the tracking function.

' An object or target may be located by giving the distance of the target from a reference point or origin, the angle of elevation from a horizontal reference plane subtended at the origin, and the angleof azimuth from a vertical reference plane. In a system for determining these coordinates to which this invention relates, pulses of radio frequency energy are radiated from a directional antenna and the pulses which reach the target are reflected therefrom and picked up by the antenna. While it is somewhat preferable to employ a single directional antenna, separate transmitting and receiving antennae mechanically coupled may be employed if desired.

To direct the radiated pulses to the target so that echo pulses from the target will reach the antenna, there are provided a motor for driving the antenna about a horizontal axis tochange the direction of the radiated beam in elevation and a second motor for driving the antenna I about a vertical aXis to change the direction of the beam in azimuth. The angular displacement of the first motor shaft from a reference position is therefore a measure of the elevation of thetarget, and the angular displacement of the second motor shaft from a reference position is a measure of the azimuth of the target. The range of the target is proportional to the delay interval separating the echo pulses from the corresponding transmitted pulses and a Suitable 4 Claims. (Cl. S18- 440) range indicator is provided for measuring the delay period and thereby indicating the range.

The present invention is applicable to a system for tracking or following a target with respect to each of a plurality of coordinates simultaneously to continuously determine the target position. However, since similar apparatus may be used for tracking the target in each coordinate, the invention will be described herein as specifically applied to tracking the target to continuously indicate its range.

In a specific embodiment of the invention herein shown and described for the purpose of illustration, there is provided automatic range tracking apparatus for detecting errors of the range indication produced by a range indicator and for setting up an electric signal for controlling the energization of a driving motor coupled to the shaft of the range indicator to cause the shaft to be displaced in such a direction and by such an amount that the range indication error is corrected automatically. Because ofthe high frequency of recurrence of the pulses which control the automatic tracking apparatus, the range indicator servesto continuously indicate the range of the target, which range may be changing at varying rates.

For initially locating a certain target and vfor tracking the target during intervals when the automatic tracking apparatus fails to function due to fading ofthe echo pulses, for example, there is provided a manually controllable tracking apparatus for energizing the driving motor for the shaft of the range indicator. In this apparatus there is provided means for generating and impressing upon the range indicator'drive motor a voltage having a component proportional to the angular displacement of a hand- Wheel shaft under control of an operator and a component proportional to the rate of change of hand-Wheel shaft displacement with the result that the motor driven shaft displacement has a component proportional to the time integral of the hand-wheel shaft displacement and a component proportional to the hand-wheel shaft displacement. `The angular velocity and phase of the motor driven shaft may thus be readily manually adjusted to cause the range indicating apparatus to track the target, that is, to continuously indicate its range. There is also provided means under the control of the operator for changing the ratio of the two voltage components applied to the tracking motor which drives the shaft of the range indicator to facilitate target tracking by manual control under various operating conditions. For example, it may be desirable to use a different ratio of voltage components in tracking a target which is accelerating or decelerating rapidly in a particular coordinate than that used in tracking a target the interrupting a rst circuit for supplying energy?" from the automatic range tracking apparatusto the range indicator drive motor and completing a second circuit for supplying=energy.trom.'` the manually controllable apparatus'A tot thev range tracking motor. When the automatic apparatus is subsequently capable1=offyperforming` the tracking function, the second circuit is interrupted and the first circuit completed-under automatic control. Tliereais provided@ a motor coupled tothe hand'- wheelf. shaft upon Whichis impresseds during automatic tracking periods, voltage-'proportional -to the difference between: the voltage :bteing supplied to a circuit for energizing'tl'ie range indicator driveL -motor fromith'e automatic range trackingunit and the voltagelwhch wouldbe supe plied to that circuitg from the manually: controllable tracking apparatus: if the systeml were conditioned 'for' track-ing byr-the manuallyl controllable apparatus.; Thev voltage,- output.. from th'ei manually controllable apparatus. isf thus maintained equalf to the voltageY output from' the automatic tracking apparatus so that the speed of the tracking motor will be'. the same immediately afterl switching to-` the manuallyK con'- trollable apparatus asvit'- was justprior tothe time of switching. Therefore, it the rate: of change otra-nge# of a target remains constan-t during a fading intervalV of thefechofpuflses, the automatic trackingapparatus' will resume control after the fading interval without the necessity of manual adjustment of the hand-wheel.l However, if the' rate oi" change offrangel variesfbyia ,substantiall v amountv during thefading period, manual adjustment of fthe liand-wheeil` tol cor'- rect the range indication is requiredl before'` the automatic apparatus will resume control'.v

In'dthe accompanying drawing, 4 Fig. 1 is` aI ydiagrammatic View -off af` tracking system in accordance with the presentinvention;

Figs.'-2 and 4, inclusive, are diagrammatic views of apparatus which maybe used in various 'embodiments of the inventiorifatfn@ producing an alternating voltage which varies inl amplitude in accorda-nce with displacement A'of' 1i-controlling shaft and in phase in accordance with the--dir rection of the shaft displacement;

Figs. 5 to 10, inclusive-,- arediagrammatic views of modiied arrangements* for ycontrolling of another portion of the automaticsrange: detector shown in Fig. 13': and

Figs. 16 and 17 are-diagrams usedl in describing the operation ofthe automatic range detector of Figs. 13,1 ligand 1'5.

Referring nowvtothe drawing, there, is shown.

inv Fig. l a system for trackingA orA continuously following an object-or target to determine its l'ocation with respectY to` azimuth, elevation and range. There are provided'an'oscill'ator I4' for generating a sinusoidal wavewillich-is;v impressed" upon a starting pulse generator I5 which generates a series of pulses of brief duration, one foreach-cycle' of the sinusoidal? WaveV The starting pulses are impressed' upon a radio transmitter Hl which produces a series of brief pulses 'ot radio frequency energy. These radio pulses `are impressed through a suitable coupling arrangement or T-R box i2 upon a dipole directional antenna !i from which the pulses are *radiated towardf a-target. Echo pulses reflected from' the target arepicked up by the antenna il and impressed through the T-R box upon radio receiving apparatus i3. The coupling arrangement`lf2' serves to shunt the input of the receiver I3 during pulse transmitting periods so that the same antenna mayfbevused bothfortransmission and reception of pulses. The-T-RV box. is a trans- Y mit-receive switch which may be of 'any desired `the displacement of a lshaft driven by an electype and may conveniently be a-Western Elec-r tric C'ompany 709A vacuum tube. YThis vacuum tube is essentially aY resonant cavity iirlled with an ionizable gas. During reception,A with thelow voltages of the received. energy, thegasis lnot ionized, the cavity is tuned to vresonanceV and the received-energy' is conducted to Athe radio receiver i3.. During `the transmission of a pulse, the voltages due to the pulse-ionize-thegas thus detuning the cavity and; preventing the energy of the pulse from: reaching the.` radio receiver i3. Means, not shown,` are providedv for continuously rotating the-dipole of they directional antenna to provide for conical scanning so that the difference between the direction of pointing of the antenna and-the actual direction of the target may be detected and;` aisignal gener;- ated for driving the antenna into asubstantially correct pointing position.

Apparatus is provided for detecting the error in pointing of the antenna H with; `respect to elevation and for producing a signal yfor encrgizing` a motor differ driving the antenna -structure/about a horizontal axis to changethe-di.- rection of radiation of the antenna ywithrespect toI elevation., Similar apparatus l2 is' provided for detecting the error in pointing-of thev .antenna Il with respect to azimuthand for producing a signal for energizing a motor d'ior driving the antenna structure about a'vertical axis toI change the direction of radiation with respect to azimuth. The antenna H is thus driven into such a position with respect to elevation and azimuth thatv the radiated pulses reach a desired target, are. reflected therefrom, and the reflected echo pulses picked'up' by antenna H. The displacement-of the driving shaft lili'I of elevation drive motor di from a reference position, as may be indicated by a suitable indicating device coupled to the-shaft, is therefore a measure of the elevation of the target and similarly the displacement of the dri-ve shaft 46 of azimuth drive motor i2 froma reference position, as mayv be indicated by a suitable indicating device l coupled to the shaft, is therefore a measure of the azimuth of the target.

In addition to the apparatus for trackinga target with respect to elevation and azimuth there is provided a range tracking apparatusk for producing an indication of the range of the target corresponding to the angular displacement of .a shaft 'Z9-coupled to a two-phase induction motor 50: through a centrifugalv clutch 51. For causing the shaft 2S to rotate rapidlyl in one direction or the other for use= in searching for a target, for example, a slew motor 52.is coupled to shaft 29'throug-lmclutch-` 53 and gearsh 54,. the

"eiergization of the range tracking motor 50, it4 i',

suffice to describe only the range tracking apparatus more in detail.

There is' provided a range indicating apparatus "Iwhich may be like that disclosed in United 'States` Patent No. V2,422,205, granted to L. A.

``lvleacham June 17, 1947. As disclosed in said patent, the range indicating apparatus comprises `aI'start-stop circuit I6 which produces a square *wave (c) in response to each pulse (a) supplied A'tofthe circuit from starting pulse generator I5.`

`Tl'ie `voltage wave (c) produced by the starts'topcircuit I6 is impressed upon a timing wave lfg'enerator 20 which generates a succession of trains of constant frequency oscillatory waves idlthe phase of which may be shifted continulusly `througha plurality of cycles in response 4tc'irotation of shaft 29 of phase shifter 2l. The 4period of this oscillatory wave, or a phase'shift of 'thfe wave through a single cycle, corresponds tofthetime interval required for a radiated pulse to travel through a certain distance and for its "echo' to return through that distance. The disjtance represented by a single cycle of the osi cillat'ory' wave is the velocity of propagation of fthe radiated'pulse divided by twice the frequency LTof 'the oscillatory wave. The timing wave (e) from the output amplifier of the phase shifter l vZ-I is impressed upon a timing pulse generator 22 which produces alternate, positive and negae tive" timing pulses (f), a pulse being produced at the beginning of each half cycle of the timing lwave. The square wave (c) from the start-stop circuit and the timing pulses (f) from the timlingpulse generator are impressed upon a pulse f'selector 23 which selects a pulse (i) of each *group of timing pulses (f) which selected pulses Vars" delayed by a desired interval with respect ttothecorresponding starting pulses (a). Echo pulses `(7') from the output of the radio receiver 1 f3'fare impressed upon one of the vertical de- "rflecting plates of cathode ray tube 3!) and the 'range pulses (i) are impressed upon the other vertical deflecting plate. A linear sweep wave (-hlis applied from a suitable sweep wave generator 35 to the horizontal deflecting plates of cathode ray tube 3D, starting pulses (a) from startirigpulse generator I5 being supplied to the sweep wave generator for maintaining the sweep "Wave in synchronsm with the starting pulses.

The rotation of shaft 29 causes a range mark` lproduced upon the luminescent screen of the 't cathode ray tube due to the range pulses (i) to travel across the screen. When the range niark on the cathode ray tube screen due to the range pulses (i) is in alignment with the mark 1"produced upon the screen due to the echo pulses zii), therange of the target may be read directly wfrom arevolution counter or indicator 32 at- "xtached to the shaft 29, the indicator being calibrated in units of distance.

The' motor 5u for driving the shaft 29 is a two-phase induction motor having a cylindrif'c-al armature which rotates at a speed propor- .A .tional to the amplitude of the alternating voltage impressed upon one of its phase windingsfroma `speed at which the range is changing so that theroutput of an amplier 60 and which rotates in a direction corresponding to the phase of that voltage with Vrespect to that of the fixed phase voltage (not shown) impressed upon its other phase winding. 'There is provided an automatic `range detector 6I upon whichthe range pulses "tiornthis apparatus'produces a current for energizing a relay 62 the operation of which cornpletes circuits for supplying energizing current from grounded battery 63' to the windings of relays 64, 65 and 66. The operation of relay 65 completes a circuit from the automatic range detector 6I through amplier 61, right-hand contact'of relay 65, Iand amplifier 60 to motor 50 to impress uponthe motor a voltage of such phase that vthe shaft 29 is driven in a direction to cause the pulses (i) to be brought into coincidence with corresponding echo pulses (j), at which `time the range indicator correctly indicates the range of the target from which the echo pulses are being received. If the target is moving so that its range is changing, there is applied to the motor 50 from the automatic range detector 6I a voltage proportional to the theltarget is automatically followed or tracked with respect to range Iand so that the range indicator Will continuously indicate the range. The

`energization of relay 64 completes a circuit for energizing a lamp 68 from an alternating current source 69 to indicate to an operator that "the automatic range tracking apparatus is functioning.

f- When the automatic tracking apparatus 6I ceases to function vfor some reason, for example,

due to the fading of the received echo pulses,

relays 62, 64, 65 and 66 become deenergized and, as a result, the indicator lamp B8 is extinguished and a circuit is completed from the output of a summing amplifier IU to the input of amplifier BIJ for supplying energy to motor 50. The summing ampliiier is an amplifier upon the input of 'which the vector sum of a plurality of component voltages is impressed. If desired, the summing amplifier may be ofthe type disclosed in U. S. 2,401,779`to K. D. Swartzel, Jr., granted June 1l, 1946. Under this condition the energization of motor 50 is controlled in accordance with the angular displacement of a shaft 1I @which may be manually controlled by turning a hand-wheel 'I2 secured to the shaft. The shaft II vdrivesran induction generator 13, the con- `-struction of` which may be similar to that of motor 5u, to produce an output voltage propor- `tional to the rate of change of displacement 'of the vshaft 'II.` Since the voltage produced by generator I3 is proportional to the speed of shaft 'II, it is sometimes identified as a tac-hometer generator. There is also coupled to the shaft 11| through gearing I4 a generator l5, such as a"synchrogenerator, which produces an alternating output voltage the phase of which is reversed in response to a reversal of the direction `of rotation of shaft II from a reference position andthe amplitude of which is proportional to the displacement of shaft 'II from the refer- 4 ence position at which the output voltage is zero.

Means such as potentiometers 'I6 and 'I'I are provided for-changing by manual control the out- Sfmotor 5&5 is coupledto shaft 29.

f1 rput 1voltage .from VgeneratorsnflS :and @15, 'frespefrtively, so `that theratio ofthe ytvvo voltages fmay-be controlled vas desired. `The .output voltage from lgenerator 15 andthe output voltage-troni gener- `tator "i3 are impressed upon the input -of sum v'fming amplifier-lll, this amplier producing =an l"output vvoltage proportional to the vector `sum -of the component voltages impressed upon its input -icircu'it. The circuit from'generatorw'l to ampli- Itfier 'lll is completed through a contact of relay y'66. A third voltage is impressed -upon the linput of amplii-ler l!! from a :tachometergenerator 18, vlike the generator "13, which producesa voltage proportional tothe speed of shaft `29 When' the This rthird `voltagefimpressed upon summing amplifier l0 isop- .posite in `phase to, vand has an amplitude vless than, .the `voltage from generator'l and `is 'prolTvided'for correctingfor inaccuracies in displace- "ment of shaft '2e in response tothe voltageim- Vpressed upon lmotor il. It is'desired that the `irate'of change of displacement of 'shaftZS'be' at -all y'times proportional to the 'voltage impressed .upon the motor 5G. When this response lis too small, .'ior example, the output of generator 't8 is VArelatively.small and 'the-energization of Vmotor 50 fis increased to correct for the inaccuracy in re- *During periods When `the automatic :range dev"tector 'fl 'is functioning to supply'energy to Jmo- "".to'r 50, a circuit is completed fromthe apparatus fll 'through amplier 67 and right-hand contact Iof relay t5 ktoJone input terminal'ofra differential `amplier El? and the output ofamplier "l iis conf'nected to another input terminal of diiierential amplier 8G. v'lhe output voltage of 'differential amplifier 80, .which Yis proportional to the -dii- `'terence of the `voltages from ampliiers f' and l0, is impressed upon a motor 8| `coupled to the hand-wheel shaitll. Under this operating con- Vf'diti'on the vcircuit for supplyingvoltage fromigenerator i3 to summing amplifier l0 is vinterrupted at'the Ycontact ofrelay 66. Whenever the voltage from the Youtput of Yampliner'l differs from the "ivoltage `at the loutput of amplier'lil, 'the vmotor "'84 :is energized to cause the rotation of shaft "'ll in tone `direc'tion'or the other 'until the two voltiffag'es'are equal. Thereforaif it be -assumedithat the range of a target is changing at a constant frate, Va constant amplitude alternating voltage will be impressed upon motor 50 fromthe autonmatic range `detectorl and, when fading of the Vecho pulses occurs With the Vresult .that relays 62, c$4,155 and Sli become deenergized. constant fvoltv"age ofthe sameamplitu'de and phase will be supplied to motor B `from generator 15, no manual "adjustment of the hand-wheel 'i2 being required.

`When fading `of vthe echo pulses subsequently '.'ceases relays 62,"54, 65 and 66 become venergized and the constant voltage is again supplied vto :motor 50 from the `automatic rangefdetector'll.

r,llfto loe energized so as to rotate shaft l29 :in ya.l .direction corresponding to the direction of rotation of the yhand-Wheel sha-ft ll. Thedriven .shaft 29 is thus given two components of rdis- .-placement, one proportional .to the-displacement lofthe' hand-Wheel shaft, and the .other proporlouter legissimultaneously decreased. Windings 8 l.tionaltto fthe --time .integral -of the :displacement -of fthe hand-Wheel shaft. The ratio y,of the -ztwo .components Aof the displacement of shaft .Zemay -be varied by manually adjusting the potentiometers .16 and ll, the preferred adjustment .for :a given operatingcondition being best determined yby trial. `For example, it ymay be desirableto have Vthe ratio lof the output from` tachomet'er generator 'i3 `to that from synchrogenerator J5 relatively largerwhen the rate of changeoffrange of the target varies rapidlythan is the case when the rate of change ofrange 'varies slowly.

Fig. 2 shows vthe synchrogenerator or trans- Aformer T5 of Fig. 1 more in detail. Current from an alternating-current sourcek is supplied-'itc 'the stator .windings 83 vof Athe generator 'l5 with the -result that there isinduced in rotor Winding 84 an alternating voltage having the frequency -of the source V85. The position of the .rotorlfis vchanged inaccordance with the angular displacement of shaft l. Atfa "certain reference position of shaft lll no voltage .is 'induced in winding 84. Displacement of .shaft 'H in one Idirectian from the reference..position causes a voltage of .one phase-tolse induced in Winding-84 and-af'shaft displacement in the opposite Vdirection-ffrom-the reference .position causes `a'voltage yof @opposite phase to vbe induced 'in Winding-84. The famplitude of the voltage 4induced in winding 84 is proportional to the displacement of shaft lLifrom the .reference position.

:In the arrangement Ashown in Fig. -3, there ris employed a magnetic core member v8l having three legs and a movable `magnetic :member-138.1 which may Ibe Amoved by vturning the -screW-88 in Iengagament With nut 3:?. As the 'screw -is turned to :move the member 6l, the reluctance of the -magnetic .path including the centerV leg and one of theouter legs is increased While the reluctance ofthe path including the middle leg and'the other .Sill and 9| on the' outer legs `respectively set I-up aiding -magnetomotive forces in the core 'so that zero flux flows through the middle leg and zero voltage is `induced in the output Winding 92-..on the -middle `leg when member 8l is Vin Va certain reference `position such that the `reluctancesof the ytvvo lmagnetic paths each including thelmiddle leg are equal. When -the member lfis moved in one'direction from the reference position, -voltv.age of one .phase is induced in Winding V92,.an'd .when lmember ill' is moved in the opposite .direction from the reference positionthe phase of the induced output voltage is reversed. The ampli- `.tude ofthe induced-voltage is determined bytlie .ratio of the'reluetances of the ltwo 4paths 'and therefore by the displacement of member v'8l-.and

Vofscrew 88 from a reference position.

The arrangement of Fig. lis'similar to that of Fig. Band the corresponding parts .bear the same designation. In this arrangement there `is 5emyployed a Y-shaped'stator lmember 0n Which` the windings 90, 91 and 92 are wound. The 'movable vmemtwerorrotor il is in the form Vof an open ring which can 'be rotated about its center by means of thehand-wheel lT2. In 'this embodiment'the legzof the Y on which WindingfQZis'wound is common to the -two magnetic paths includinglthe other legs ofv :the vY, respectively, 'the reluctance of one of which is increased While that ofthe other fis vsimultaneously decreased due to ytherotation' of hand-wheel l2.

-In Fig. 5 is shown Aa modified arrangementior controlling the'angular displacement fof afshaft v#L00 driven -by ay reversible motor :1.01 under :con-

azafatas The brush |06 is connected to one terminal of a` circuit` including a resistor |01 and a condenser |08'in shunt with respect to each other, and the other terminal of this circuit is connected to one terminal of motor the other motor terminal being grounded; It is seen that one component of` the energizing current supplied to motor |0| flows through resistor |01 while the other component iiows through condenser |08. The direction of the current flow through resistor |01 depends upon the direction of the displacement of brush |06 from the grounded mid-terminal of the potentiometer winding and therefore upon the direction of the displacement of the shaft |02 from a reference position. The amplitude of the current through resistor |01 is proportional t0 the angulardisplacement of shaft |02 from the reference position. The amplitude of the current iiowing through condenser v|08 is proportional to the rate of change of displacement of shaft |02.

The speed of motor shaft |00 is assumed to bei proportional t0 the voltage impressed upon the motor. Therefore if H is the displacement of the controlling shaft |02 and 0 is the displacement of `the driven shaft |00 and E is the voltage impressed upon the motor |0|,

j `I` p0=BH+ApH f 0=B%+AH and ,l E=BH-|ApH Where A, B, A' and B' are constants and p is the differential operator.

2 In Fig. 6 thereis shown a modification of the arrangement depicted in Fig. 5, the similar parts having `the same designation in each figure. In this embodiment there is used a two-phase induction motor i0 for driving the shaft |00. One phase of a two-phase voltage source is impressed upon one winding of motor ||0 while the second phase ||2, in quadrature with respect to the first, is impressed upon the winding of a polarized relay ||3. A terminal of the network comprising resistor |01 and condenser |08 is connected Ythrough the armature and contacts of relay ||3' to the end terminals alternately of the primary winding H4 of a transformer, the secondary winding l5 of which is connected to the second winding of motor I0. A mid-terminal of the winding ||4 is connected to ground. In this embodiment of the invention, the direct current output from the potentiometer |04 and resistancecapacity network |01, |08 is converted into alternating current by means of a relay modulator. The use of the push-pull connection eliminates all even harmonics of the fundamental frequency and,rif the relay ltiming is so adjusted that each contactclosure lasts for one-third cycle, and each `open period of the contacts lasts for one-sixth cycle,k the output signal contains no harmonics which are multiples of three. The lowest harmonic `.present in the output signal is therefore the fifth and a shunt condenser to reduce this harmonic can readily be tolerated.

VThe embodiment of the vinvention shown in Figry'? `diiers from that showninFig. 5 in that one phasei l` of a two-phase source is applied'to an induction motor ||`0, while the second phase ||2 is applied across the potentiometer |04. In place of the resistance-capacity network |01, |08 of Fig. 5 there is used a network comprising resistor ||6, condenser ||1, and inductance element |l8, all connected in parallel. The portion of the network consisting of condenser ||1 and inductance element IIS is anti-resonant at the power supply frequency just as condenser |08 of Fig. 5 has infinite impedance to direct current.

In the embodiment `of the invention illustrated in Fig. 8, there is coupled to the hand-wheel shaft |02, in addition to the brush arm for moving brush |06 of potentiometer |04, a tachometer generator ||9 which generates a voltage proportional to the speed of shaft |02. 'Ihe voltage from potentiometer |04 proportional to displacement of shaft |02, and the voltage from generator i I9 are connected in series to a winding of motor |10. One phase of a two-phase power source is` applied to the other phase winding of motor ||0 while the second phase of the power source is applied across potentiometer |04. The motor l0 and generator ||9 may be of similar or, for low output power, identical construction. Particularly suitable is the so-called drag cup unit which makes use of a two-phase winding on the stator and a thin shell of aluminum or other suitable conducting material for the armature. One of the stator windings is 'excited continuously at full rating.' If a control voltage is applied to the second stator winding which is in quadrature to that applied to the `first winding, a torque substantially proportional to the control voltage is" produced. `If instead of applying a quadrature lcontrol voltage to the second stator winding the rotor'is driven mechanically, a voltage proportional to rotor speed is generated in the second stator winding.

In Fig. 9 the displacement of an output shaft- |20, driven by a motor |2| through centrifugal clutch |22 and gears |23, is under control of va controlling shaft |24 which may be angularly displaced by rotation of a hand-wheel coupled to shaft |24 through gearing |26. shaft |20 may also be driven at a rapid rate independently of control from shaft |24 by means of a motor through centrifugal clutch |5| when an energizing circuit for motor |50 is completed from source |31 through a manually operated key |52. Theshaft |20 may be coupled to a range indicatonffor example. The shaft |24 drives a brush |21 in sliding contact with the winding of a potentiometer |.28 one terminal of which is grounded and the other terminal of which is connected through a resistor |20 to the positive terminal of battery |30 having its negative terminal connected to ground. provided a modulator comprising electronic devices |3| and |32 each having an anode, a cathode and a control electrode. The potentiometer brush |21 is connected to the control electrode of tube |3 Current from source |30 is also supplied through resistor- |29 to the voltage dividing resistors |33 and |34', one terminal of resistor |34 being groundedand the common terminal of resistors |33 and |34 being connected through variableresistor |35to the control electrode of tube |32. A Variable resistor |36 has one terminal connected to the brush |21 and its other tera phase shifting condenser |38 and a resistor |39, -aterminal of resistor |39 and a terminal of source |25 The There is agsreg'zes 111'." 31 being grounded.` It will loe-seenA that; there. isincluded; in. the controlI electrode-cathode circuit of each. of tubes ISI' and |32 `an' alternating voltage and a direct voltage. The alternating voltagey impressed upon yeachof these circuits is that: appearing acrossresistor due to current from source |31 flowing therein. The direct voltagefcomponent in the control electrode-cathode circuit/of tube |31 is the voltage drop acrossv that portion of potentiometer |28 between the brush |.2'1and ground. The directvoltage in the control electrode-cathode circuit of tube |32 is the voltagedrop acrossresistor |34 to which is added the. voltage drop acrossresistor |35 when current flows through that resiston. For supplying anode current. totubes l3|and1 |:32 the positive terminalofbattery lqisconnected to a mid-terminal oflprimary winding. of transformer |40, the end` terminals of this primary winding. being connected'tothe anodes of tubes |3|s and |32, respectively. Whenthe voltage drop measured. be tween the brush |21 of potentiometer |28 and ground is equal to. thevoltage drop across resister |34, atwhich time it may be saidthat. the shaft |24 isin a reference position, the resultant. voltages applied to the control electrode-cathode circuits of. tubes ll and` |32, respectively, are equal; so that the anode currents of tubes |3| and-|52 flowing through the two` halves of the primary winding of transformer |40 are equaland the voltage across the secondary winding of transformer |40.. is zero.

Assuming for the present. that no current. is

flowing through resistor |35, when the'shaft` |24v isangularly displaced, the direct voltage component. impressed. upon. the control; electrode?- cathode circuit of tube |3| is increased or'decreased, depending. upon the direction of thev displacement, by an amount which; varies with the displacement while the voltage impressed upon the control electrode-cathode circuit of tube, I32.remains fixed. There is thus produced acrossv the, secondary windingA of transformer |40a volt agethe phase of which changes with the direction 0I; rotation of shaft |24. and the amplitude, of which varies Withthe amount of the displace mentfrom the reference position. The voltage produced across the secondarywinding of. trans-f former |40 isimpressed upon a push-pull amplifier comprising electronic devices |4;| and |42. each having an anode, a cathode, a control. grid and. a screen grid. The control grids of tubes;

I4|L and |42 are connected respectively to the end terminals of the secondary winding of trans-4 former |40 while the cathodesof the*V tubes arei connected through a varistor |43 to the grounded mid-terminalof the secondary transformer winding. Screen grid voltage is supplied to Ythetubes For supplying anode current, tothe tubes, the positive terminal of battery-|30.e

from source |30.

is connected to the mid-terminal of the. primary winding of a transformer |44, the end terminals of. which are respectively connected to the anodes The secondary'windingof. transformer |44 is connected to `one;of,Y the;l windings of motor |21, voltagein quadraturewith4 the voltage from transformer |44 being supplied; tothe second Winding of motor |2| fromsource.A The, brush |51l of tubes |4| and |42.

31 through a transformer |45. ofa potentiometer having a winding |56.is driven; through gearing |23 by the motorv whichdrives the output shaft |20. The.positiveterminaloff battery |30 is connected through a-resistor |59. to one` terminal fof winding I58;the other terminal.

of which is grounded. The brush |51 is con,ne cted;.

,.3 transformer |44.

through; a. variable:.condensery |6Ilv and 'throughrl resistor.` |35 to the common terminal of` resistors..

When; .the motor. |21 drives the output shaft |20 andthebrush.- |51 ofpotentiometer |58, the; voltage measured between the brush |51 and'y ground is increased or decreased withv respect to thevoltage acrossthe portion of the circuitcomput shaft: |20, an increase of charging current;` tendingtomake the control. gridofi tube |32 more,

positive.Y However, since .anincrease of potential; of; thev grid of tube |32 is oppositev in. its effect;

upon'A the. output voltage impressed upon.. motor'.

|.2;| to-.asimilar increase inpotential ofiY the grid'. of. tube |3.f|,.the eectivevoltage impressed upon.. the input. of. the modulator |3|, |32, under static. conditions of. the hand-wheel shaft, is proportionalltozthe difference between the displacement. of; the'handewheel shaft |24 andthespeedwith; a-suitableiproportionality factor, of output shaft..

|20. The speed of motor I2 Iriszsubstantially proiportional to the voltage impressed upon it from Because of the amplification of modulator |3I, |32 andof amplifier I4|, |42, only a very small effective input voltage to the modulator is required for causing a sufficient'y energization of themotor |421; If, for example,

the., speed: of `motor |2|i is less than its correctJ or normal speed corresponding,to.a..certain. dis.- placementof the handwvheel4 shaft, the. potential impressedupon thev grid of tube |32. is also less.V

than normal so that the effective voltageime` pressedupon the input of the modulator |3|, |32. is., greater thanznormal. Thus, the energizationy of` motor l2 I is greater. thannormalfand the motori is .brough t substantially to the` correct speed proportionalto. the hand-wheel shaft displacement. Silifiilarly,y if. the speed ofthe motorV should be. greater,.- than-.the correct speed .corresponding toa. Certainha'nd-W-heelshaft displacement, the feedf.-v backsignal impressed .upon Ythe grid-.circuit of tube [32.3 willacause.- the motor. speed to be reducedto substantially; the. correct. speedsuch..that the, speedis; substantially. proportional to the handi-Y wheel; vshaft displacement. relatively `more .accurately proportional: to hand-1 wheelshaft displacement-.when theamplication: used f ina the modulator andn amplifier 4is relatively-A large The `effective input voltage to the modu` latorundergoes changes,l automatically, to com pensateA for vacuum tube and motorV irregularities,-

Inorder. to .maintain the effective input Voltagetozthe-:modulatorat a very smallvalue, thatt is,

speed .ofi the output shaft' be maintained proper tional to. the: displacement ofA the hand-wheel shaft;A but ini addition, when the displacement.:-

of. the.v hand-wheelshaft. is changed; there must.

be a change of outputshaft displacementpropor-l tionalltoithe; change. of displacement. ofthe handwheel shaft;

The motor. speed is:v

Whenl the handfwheelf shaft .diserplacement.. is. increased: to-cause anA increase ofi atterrare thepotential on thegrid of. tube |31, for ex-u ample, the output shaft displacement must be increased by aproportionate amount to increase the Voltage across the portion of the potentiometer between brush |51 and ground and thereby causezthe potential of the grid of tube |32 to be increased due to the increased current flowing through the circuit comprising condenser |60 and resistors |35 and |34.L If, for example, the increase of displacement of the output shaft should be less than the correct or normal amount proportional to the increase of displacement of the hand-wheel shaft, the increase of potential of the grid of tube |32 would also be less than the correctvalue. As a result the energization of motor |2| would be greater than normal for the new hand-wheel position so that the output shaft would be further displaced to cause the potential of the grid of tube |32 to be brought to the correct Value. Thereafter, until the position of thehand-wheel shaft is again changed, the motor runs at a new speed corresponding to the new hand-wheel position such that the potential of the grid of tube |32 is maintained at a correct value nearly equal to the potential of the grid of tube I3 I. The over-all result is that the output shaft undergoes a change of displacement proportional to the change of displacement of the hand-wheel shaft and assumes a new speed proportional to the new displacement of the handwheel shaft. Therefore, the output shaft has one component of `displacement proportional to the displacement itself of the hand-wheel shaft and a second component of displacement proportional to the time integral of the hand-wheel shaft displacement.

i Thus the effective voltage impressed upon the modulator circuit has one component proportional to displacement of the hand-wheel shaft |24 and a second component proportional to the rate of change of displacement of the hand-wheel shaft each of these components being very small with respect to the potential applied to the grid of tube |3| or the potential applied to the grid of tube |32. The ratio of these two components maybe changed by adjustment of the resistance of .resistor |35 and the capacity of condenser |60, or both, to change the time constant of the condenser charging circuit. Moreover, the ratio of the speed of output shaft |20 to the displacement of the hand-wheel shaft |24 may be changed by adjusting the resistance of resistor |35, the capacity of condenser |60 being also changed togmaintain the time constant of the condenser charging circuit fixed, if desired.

`When the resistance of resistor |36 is infinite, the ratio of the effective voltage impressed upon theimodulator |3|, |32 in response to displacementfof input shaft |24 to the displacement of shaft |24 is constant. It is desirable in some cases to make this ratio variable, as explained below in connection with Fig. 10, and it is for this reason that resistor |36, the resistance of which may be adjustable to a suitable value, is provided.`

Toitrack a target, the position of which is changing slowly in a coordinate, a low angular velocity of the output or controlled shaft is required and when the operator observes a tracking error he desires to make only small corrections both in the component of output shaft displacement which varies with hand-wheel displacement and in the component which varies` with the rate of change ofhand-wheel displacement When the position of the target inzthe` .to the maximum output voltage EB.

of the curve is determined by the ratio of the;

cordinatef is changing more rapidly, the correo--` tions which the operator desires to introduce-V become greater. These two conditions may be illustrated by means of the curve shown in Fig. y10 f in whichthe abscissa is the ratio of hand-wheel displacement H of a potentiometer P, from a.

reference position giving zero output voltage, to

maximum hand-wheel displacement and in which the ordinate is the ratio of output voltage-En The slope resistance of resistor R to the resistance of the winding ofpotentiometer P. When the ratio R/Po is infinite, that is, when resistor R is omitted,Y

the slope of the curve is the same at low as it is at high values of hand-wheel shaft displacement. As the ratio of l-'t/PY is decreased, the slope of the curve for low values of hand-wheel displacement (So) decreases while the slope of `the curve for large values of hand-wheel disment, are eachl larger for large hand-wheel disments than they are for relatively small handwheel displacements. ponents, however, remains unchanged. If Fig. 8 is modified to include a resistor R having one terminalconnected to brush i |06 and `its other terminal grounded, vthe rate of change of the,

voltage component varying with hand-wheel displacement, with respect to hand-wheel displacement, changes, the Voltage varying with rate of change of hand-wheel shaft displacement produced by tachometer generator ||9 being independent of the position of the hand-wheel shaft. In this case the ratio of the two components of the -voltage impressed upon the driving'motor ||0 changes in response to a change of handwheel shaft displacement.

Fig. 11 depicts a. generalized tracking arrangement for causing a displacement 0 of an output i.

shaft having a component proportional to the displacement H of an input or controlling shaft and a component proportional to the time integral of the displacement H. Specific .embodi-.

ments of the tracking system depicted generally in Fig. '1l are shown in preceding figures. The curve of Fig. 12 shows the frequency gain characteristic of a tracking arrangement of the type depicted in Fig. 11. between H and 6 is given by the equation 0`=AH -l-Bf H di where A and B are constants which may be' respectively called the displacement ratio and ther rate ratio. In operational notation this equationl beCOmeS ratio of therate ratio B to the displacement ratio A and is the reciprocal of the so-called time con- It is equal to thev stant of the tracking system. frequency ,in radians per second or equal to'21r times the frequency fo in cycles per second at* which the slope of thecharacteristic depicted The ratio of the two com-A The desired relationship germes:

InFig. llgr'lw represents means for converting.` frorrr. a. shaft. position H? to an: electricV signal= fcllnwedlby: an: electric network, u representsl a`- network` followedfby-avdevice such as an electric motor for convertingfromz an. electric signal. to alshaftslposition 0, and isa feedback element whichconverts from a-shaft'position Hfto-an electric; signal. Whichis: impressedl upon the 'input of thenetwork; ,a The elements ganci may/haveaicharacteristic depending upon` frequency. The over-all. characteristic of such a system is--given bym the equation'` Moll'.

If .We ignore the constant A,

L' .Ji. 1 l0 H l-'M andthe problem of obtaining a tracking systemhaiving: the. desired characteristic depicted'A inr Fig; 12. is-one of designingpap and so that To. obtain.. stability it is desirabley to make p. much greater than unity throughout the useful. frequency bandA which in a manually aided. trackingsystern extends from` direct current to a fre.- quenc'yv usually somewhat. lessthan. one cycle per. second; Under this conditionthe above equation reduces toA trim l'A P' Thr-eeA sets of values for ,un and' ,8 such `asv to ysatisfy' thisA equation are given below.

frequencyr responsel characteristic and thefeed bacleele'ment ,Bhas a characteristic which is the inverse o-that desired. A specic embodiment ofisuchv anarrangement is shown in Fig. 9.

In"Y this :case both: thel inputY element. fw. and. the.

feedback element ,8 contribute tothe. desired fre quency; response'-characteristic'. A specific. erny bodiment ci thisI type -of system isshownzinFig. 1 where the characteristic 0:10-1-10 is produced by the tachometer generator 13 and the synchrogenerator. 15 and 1 where the Vcharacteristic ,Sz-p issproduced by thertachometer generator 18.

AThe;automatic range detector 6l will now be described in detail with reference to Figs. 13,.'1.4 f,: 15, 16 and 1'7 which correspond, respectively, to a portion of Fig. 1 and-toFigs. 2, 3, 4 and 5 of application Serial No. 5232721 to B. M. Oliver, filed February 24, 19.44.

` Theoutput pulses (il from: the circuit I.9 are; applied to a delay circuit and pulse` generatore.. 22|.; which; for. example, produces az pulse ,308:1 (see Fg;.16-G) Which has ay duration correspond-x ing to' a Vrange ofi,- for example, 40.0 yards,4 ther; pulsef being .initiated after a4 time. interval.

- iny Fig. l'C) whichisapplied to .thegating Wavev 75x scribelzl;4 below;-

generator 2231. The pulse 302' has a time dura@ tion corresponding: to 1200 yards range, forexa` ample; andxthis. pulse is started' bythe rangeunit pulse. 301.1` Fig. l-D shows tworgating. waves 303;' and. 304': which.v are produced'by the generator 223; The-waves303 and S'llllareappliedv to arange'detector 225th' which is'. alsol applied" th'ev selected portioniof the video signal fromy the sig-1 nal selector 220; rIlhis selected signal is repre. sented-by; the pulse-..309 in Fig. 1li-H. andV is pro' duced byl applying! a wave such asv that shown inl? Fig. 16-F (representing.y received' and: detected pulse l 3D@ corresponding. to the transmitted pulse, 1 variousechoes Sill' and noise voltage components to the. signal selector 22D along with the 40u-yardl positive orV pedestal pulse 338. The pulse 3118'- causesthesignal'selector220 to pass current for theduration. of this pulse and reject all portions. ofrthe signal. produced.' by the receiver |3f which. do; notoccurwithinA the time span of thev u-yard pulse1308'.; This. isz represented. in Fig.; 16.H,. the'selectedf echo: signal being shown bythe pulse 309. In other words, the signal selector'22ilvhas an'. output current. only during the time.. span'. ofl the; pulse'v '3508y andv the position: of v` this z pulse4 withfrespect to.` the pulse'. 3110'- is deter:- 1

mnedzby'the position of.v the range unit pulse 30| with. respect tot thepulse '3118i' The range detector 225: whichzwill; be more fully'described be-l low` inf'. connection. with'v Fig. 141i` comprises tvvov diod'es to the plates of which are applied? respec tively the. gating Waves'v 303i' and. 384 from theamplier3224 and tothe plates' of both of which. isyapplied the.- selected; video signal from; theI cir-vy cuit 220i.: Integrating condensers are connected? to the cathodes of. the diodes and voltages arey produced .therea'cross which are respectively representative ot? the total. currentl passed by the diodes during the positivey halves ofthe waves 3ll31and1304f-by means vvhichv Willbe described' below. Ifithe pulse 309" is notsymmetrically'posi'-4 tioned in time with yrespectzto the` gating WavesV4 3ll3andi34 (it has: been shown as being symmetrically positioned with respect tothe mid-'fv dlerpositivepulses of-the waves 303- and 331i)` av differential current is produced which is utilizedto driveta motor (Fig. 1)- to control the range" unitl|.9zin;such1a- Way as to vary the position of thezrange unit pulse 30| shown in Fig, 16`B with respect to theA pulseil'll shown'in'Fig. 16-A. *The*- current used vto drivel'the motor 50 is produced in the modulator 226 to which the signal current@ from. .therange detector 2254s applied-in 'addition toI GO-cycle Waves from the source 23521.01:- ing through a phase shifter 229.Vv The output of` the-.modulator 226 which is a -cycle Waveamw plitudeim'odulated by the signals fronrthecircuit'4 225 is` amplied in' the amplier 6l and applied tothe motor llfthroughan amplier 6U and aV contact and. armature of control relay which latterzisaoperated::by means'oflrelay 62, as cle-- l Fig. 154 shows the pulse generator 222, the 410'- kilocycle generator 22.3 of the two gating waves, ampli-fier 224., range detector 22.5 and direct coupled amplifier 225A. The pulse generator 222 comprises a tube V to the control grid of which is applied through a resistor 242 the positive pulses 30| (one for each transmitted pulse 355) from the range unit i9. The cathode of this tubeisv connected to the suppressor grid and also to ground through the parallel connected condenser 24| and resistor 242'. The cathode is also connected through resistorsv 24.3'` and 244 to the positive terminal of a source 245 oi constant potential of 300 volts, for example. While the source 245 has been represented schematically as a battery, the negative terminal oi which is connected to ground, it is to be understood that any other suitable source can be used. The screen grid of the tube V| is connected through the resister 244 to the positive terminal of the source 245 and through a condenser 24S to ground. The plate of the tube Vi is connected through the resistor 24.1 to the positive terminal of the source 245 and through a condenser 248 and a resistor 249 to the grid of the tube V2 in the generator 223. Connected between the common terminal of the condenser 245 and the resistor 24E and ground is a tuned-Y circuit T comprising a parallel connected inductive member 25!v and condenser 252;: the latter preferably being adjustable and only a few micromicrofarads. The control grid of the tube Vi is biased below cut-off by placing the cathode at a positive potential by means oi the resistors 242 243 and 244, these resistors acting as a voltage dividing potentiometer. When the range unit pulse 30| is applied to the grid of the tube Vi, this tube conducts plate current for the instant that the control grid is above the cut-oli voltage. The pulse of plate current drawn by Vl` during the time of the range unit pulse 30e! charges the condenser; 25| through the path comprising this condenser, the condenser 248, the plate cathode resistance of` VI, and the condenser 24|. This causes the plate voltage to drop about Zilti'volts and since condenser 248 .is much larger than condenser 25|, the grid of the` tube V2 is driven negative by the same amount. The VLC network comprising the members 250 and 2.5L begins an oscillation which is quenched after one-quarter cycle because the voltage across it begins to swing positive and the grid of tube V2 begins to draw current. The voltage ap plied tothe grid of the tube V2 has the wave form shown in Fig. lG-C. The period of this oscillation can be adjusted by means of the condenser 25|; which is adjusted to make the duration of the pulse 3632 in Fig. 16-C equal to a period of time equivalent to a L200-yard range, for example. The large negative grid voltage pulse 302 cuts off the tube V2 for a length of time equal to substantially one-fourth of the period of one oscillation of the network T and produces, by means of apparatus now to be described, plate voltage waves 353 and 3M shown in Fig. 16-D.

Tubes V2 and V3 collectively comprise a generator oi the gating waves 323 and 324 which are 185 degrees displaced from each other and are asfshown in Fig. 16-D. These waves are sine waves of a frequency of about 410 kilocycles, this frequency being that repired to produce three complete cycles within a period of time corresponding to a range oi 1200 yards. Associated with tube V2 is the network 252 comprising a transformer 253 having a primary winding 254 and a secondary winding 255. The primary winding 254 is shunted by a condenser 256 and the mid-point oi the winding 254 is connected to the cathode of the tube V3, the mid-point of the winding 255' being connected to ground. The winding 254 has one of its terminals connected to the plate of the tube V2 and through a condenser 25'! and resistor 258 to the control grid of the tube V3. The winding 250, has its other terminal connected through the resistor 259 tothe positive terminal of the direct current source 245 and through a condenser 26|) to ground. The terminalsl of the secondary winding 255 of the transformer 253 are connected to any suitable push-pull amplier 224. The cathode of the tube V2 is connected to ground and the cathode of the tube vV3 is connected through the resistor 26| to ground. A grid leak resistor 252I is connected in the grid cathode circuit of the tube V3 while the plate of the tube V3 is connected through the resistor 263 to the upper terminal of the' winding 254.

The grid of the tube V2 receives the 1200-yard negative pulse 352 from the tube VI. Plate current for the tube V2 normally flows through the transformer winding 254 in the network 252. The voltage pulse 322 applied to the grid of the tube V2 cuts the tube off and the change in plate current of V2 causes the tuned circuit of the network 252 to oscillate. When the grid voltage of the tube V2 rises above cut-ofi (after a period oi time corresponding to a range of 1200 yards), plate current hows again through V2, and the low pate resistance of this tube damps out the oscillations.

The tube V3 is used to supply feedback to the network 252 of just the proper amount to make up for its losses and thereby maintain a constant amplitude for all cycles in the oscillation. The condenser 25| is adjusted until the duration of the large negative grid voltage wave 302 extending below outoi of the tube V2 is just long enough to produce three complete oscillation cycles of the network 252 before the oscillations are damped out. The wave forms of these oscillatory waves 3D3 and 354 are shown in Fig; I6D. v

`The voltage waves 303 and 354 produced at the respective terminals oi the secondary transformer Winding 255 are amplified by any suitable` push-pull amplier 224 and applied respectively to the two plates of the double diode tube V4. Equal resistors 254 and 265 are connected in series across the output terminals oi the pushpull amplifier 224 and the common terminal ci these two resistors is connected through a resistor 26S to ground. The left cathode of the tube V4 is connected through the parallel con `nected resistor 251 and condenser 268 and the resistor 269 to ground, while the right cathode of the tube V4 is connected through the parallel connected resistor 278 and condenser 21| and the resistor 269 to ground. The two cathodes are also connected through the series resistors 212 and 213 respectively, to the input terminals of the direct coupled amplier 225A which is of any suitable form. Equal condensers 214 and 215 are connected across the input terminals of the amplier 225A, the common terminal of the two condenser's being connected to ground. The elements 212 and 214 and 213 and 215 serve as two low-pass iilters.

The action of the range detector is as follows: The amplified output waves of the 4l0-kilocycle generator 223 (such as those shown in Fig. 16-B) 19 are applied to the two plates of the double diode V4. An adjustment can be made in the pushpull amplifier 224 so that the voltages applied to the two plates of the tube V4 are equal when no video signal from the signal selector 220 is applied to the plates of the tube V4. The signal from the signal selector is applied to the common terminal of the resistors 264 and 295 and this f is applied equally to the two plates of the double diode tube V4. A coupling condenser 215 can be used in this input circuit if desired. 'I'he period of theselected signal shown in Fig. 16-I-I is, as pointed out above and as shown in Fig. 16 of the proper length of time to correspond to 400 yards range and this 40G-yard pulse coincides with the time of the middle cycleof the two 410-kilocycle sine wave oscillations. Once this adjustment is made, the relation between the 40G-yard pulse shown in Fig, 1li-G with respect to the waves shown in Fig. 16-D remains lixed even though the time of occurrence of all of these may vary with respect to the time of occurrence of the corresponding transmitted pulse 309,'this variation being caused by the changes in time of occurrence of the' range unit pulse 39| when the apparatus is being utilized to tgack the selected target.

If no video signal is applied to the plates of the tube V4 from the signal selector 220, the current through the two halves of the double diode V4 will be equal and will appear as two series of positive half sine waves 395 and305, Fig. lil-E, as each half of the tube V4 conducts alternately. The pulses are integrated by the condensers 26B and 2li. The resulting signal voltages are applied to the two grids of a balanced direct current amplier arrangement represented schematicalli7 by the box 225A in Fig. 14 through the lters 212, 214 and 213, 215 and in turn produce equal voltages.

If there is a video signal exactly in the center of the 40G-yard pulse shown in Fig. 16S-G, its voltage will add equally to the voltage applied to the two vsections of the tube V4 to cause increased currents to ilow. The currents through the two halves of the double diode V4 will be increased' by the same amount so that the two currents will still be equal. As before, this will result in equal signal voltages at the input of the modulator 226.

If the selected echo signal 399 occurs inthe rst 200 yards of the 40G-yard range pulse, onehalf of tube V4, say for example, the right half, will be conducting. The positive voltage of the video signal. will add to the voltage on this plate to cause an increase in the flow Iof current through this diode. The voltage applied to the left plate of the tube V4 will benegative at this instant and the selected signal cannot cause current to flow in this half of the tube. A half cyclelater the leftplate `will be positive and cause current to iiow but the video signal will not be present to add to this current so that it will be less than `that which iioWed in the right half.

Il' the signal from the target occurs in the second 200 yardsof the 40G-yard range pulse I 08, thecurrent flow through the resistor` 267 will be increased While the current flow through the resistor` 219 will remain normal. The effect is the reverse of that which takes place under the conditions described in the immediately preceding paragraph. The unequal voltages to ground at these resistors are applied to the input circuits of the balanced direct coupled amplier 225A and then applied tothe modulator 226;

The modulator 226 preferably comprises a bridge structure of four rectier elements such.

as that shown in an application of B. M. Oliver, Serial No. 491,829 iiled June 22, 1943. An alternating current from a suitable source, such as the source of Gil-cycle voltage 235, is applied through a QO-degree phase shifting network 229 to `one diagonal of the bridge, the other diagonal being connected to the output terminals of the direct coupled amplifier 225A. The modulatorv 225 operates in accordance with the description in Patent 2,025,158 issued December 24, 1935 to F. A. Cowan to suppress the carrier'from the source 235 andv transmit to the output circuit of the modulator substantially only the'upper and lower sidebands produced by the amplitude modulation of this carrier by the signal input. TheY output wave of the modulator is shown in Fig. 8B of the above-mentioned Oliver application, Serial No. 491,829.l The'output wave from the modulator is amplified by the ampliiier 91 which may be of the conventional push-pull type and by a second amplifier 69 of any suitable type. VWhen the control relay 95, in a manner to be described more fully below, is operated, to the automatic position, the ampliiier 51 isconnected through a contact and armature of the control relay 65 to the motor 59. Since the carrier input to the modulator 225 is shifted degrees by the phase shifter 229, the output of the amplifier B0 will bear a plus or minus 90 degrees phase relation to the fixed phase excitation of the motor depending on the direction of the unbalance which drives the modulator 226. Any unbalance voltage resulting from the received signal n-ot Occurring symmetrically with respect to the two gating waves (that is, with respect to the pulses 395 and 3955 contained within the span of the 400- yard pulse 399) thereby causes rotation of the armature of' the motor 59. direction to'vary the timing of the output pulse of therange unit I9 in such a way that the gating waves are centered about the received signal 399, reducing the unbalance of the driving voltage to zero.

It sometimes happens that in the operation of the range tracking unit described above and its associated apparatus, there is a fading in the received signal, so that for periods of time there are no'signals from the signal' selector. In such a situation there is no differential current to drive the motor 59. Moreover, it might occasionally happen that the selecting 40o-yard pulse 398 shown' in Fig. 16-G does not span the desired echo signal which condition mayexist in initially setting the apparatus for automatic tracking opera-y tion or when the apparatus is set into operation again after a period when it has been turned olf. In such a situation it is desirable to manually control the operation of the'range unit I 9 so that the range dial can give an accurate indication of the range to the desired target.

In order to operate the control relay 65 from the manual to the automatic position and Vice Versa, there are provided a QO-degree phase shifter 299 for shifting the phase of the gating `waves from the generator 223 by 90 degrees, an'

amplier 29| for these waves, an automatic tracking indicator and `control detector 292 (called the A. T. I. C. detector) similar to the doublediode tube V4 of Fig. 14, an am'plier 293, an automatic tracking indicator and control relay 62 (called the A. T. I. C. relay) and an automatic tracking indicator 295. made to Fig. 15 for a more detailed description of this apparatus.

Referring now to Fig. l5, gating waves 393 and;

The rotation is in a Reference Will now bey 3M shown in Fig. 16D at the terminals of the secondary winding 255 of the transformer 253'y are applied to the two 4control grids of the double triode tube V5 which `with its associated circuits acts to shift the phase of these two waves by 90 de# grees. The two control grids are connected to ground through resistors 3I and 3H, respectively, and the two cathodes are connected to ground through resistors -3l2 and 313 respectively. 'Ihe twoplates are connected to the positive terminal of the direct current source 235 through resistor 3H. Connected in parallel between the two cathodes of the tube V5 are two series connected circuits, one comprising the resistor 3I5 and the condenser 3ft and theY other comprising the condenser '3H' and the resistor 3l8. The tubeV V5 serves as a double cathode follower tube to drive the phase shifter, and the outputs of this phase shifter are taken from the points C and D which are the respective common terminals of the two series connected circuits just described.

The voltages applied to the tube V5 result in alternating voltages from points A and B to ground which are equal and 180 degrees out of phase with each other. Since this is true, the voltage between points A and B will be twice that of the voitage between either of these points to ground. These voltages are shown in the vector diagram of Fig. 17 by vectors AG and' GB and the voltage AB is the sum oi thesetwo vectors. In this diagram the point A is used as a reference point and, therefore, the arrows for vectors AG and BG are not shown 180` degrees apart as would be the case il point G- or ground were used as a point ci' reference. The currents I1 and In flowing between points A and B by way of parallel paths ACB and ADB, will lead the voltage AB by 45 degrees as shown on the vector diagram, since each of these paths has a resistance 315 or 3-l8 in series with a capacitive reactance of the same magnitude at 410 kilocycles (3960 ohms). The voltage drop Enr across resistor 3%-8' will be in phase with the current I1 as represented by vector AC. The voltage drop- Eci across condenser 3H lag-s behind current l1 by 90 degrees and is represented by the vector CB. The voltage drop Eczacross the condenser (H3 lags behind current I2 byy 90 degrees' and is represented by the vector AD. The vol-tage drop Enz across' the resistor 315* is in phase with the current Iz and corresponds to vector DB.

The resulting voltage between C and D is 90 degrees ahead of the voltage AB. On the vector diagram of Fig. 17 the voltage from point C to ground adds to that .from point` O. to gro-und so that vectors CG and DG are represented. by arrows pointing upwards. However, if point G had been used as the reference point instead of point A, the vector GD would have been shown with the direction of the arrow reversedsince thevoltage iirom point C to ground is 180 degrees out of phase with that from point D to ground. The vector voltages at points C? and D are then 9@ degrees ahead of the voltage AB at the cathodes of the tube V5y and the voltage to ground at point C is 180 degrees out of phase with the voltage to ground at point D. The voltages from the points C and D are connected to any suitable pushnpull amplifier 32B'.

These voltages are amplified and inverted by the amplifier 3:20 and applied to the plates of a double di-ode tube V6 comprising the A. T. I. C. detector 32 l, this double diode tube being similar to the tube V4V shown in Fig. 1:4. Like the arrangement including the tube V4 in Fig. 14 a signal from the signal selector 220 is applied to the two plates of the double diode tube V6. The ysignal from the signal selector 220 applied to the detector` 312i is a positive pulse which raises the voltage ofboth plates off the double diode by an equal amount. Since the the Lllll-kilocycle gating waves have been shifted degrees in phase by the action ofthe tube V5 and its associated circuits to become the waves 35E)` and 35i shown in Fig. ifi-I, the right section. of the double diode VE, for eX- arnple, willbe conducting during the middle 200 yards of the pulse 398 while the other half of this tube is cut off. Therefore, a video signal occurring within i yards of the center of the 40()- yard pulse 308 will cause an additional flow of current in one half of the'tube V6 butV not in the other half. Fig. l-J shows the current pulses 352 and 353 through the two halves of V5 due to the gatingwaves alone, Fig. 1li-K shows the cur rent wave 354 through one diode, Fig. lG-L shows the current wavethrough the other diode when the selected echo 339 is within the middle 200 yards of the 40G-yard pulse 30.8. However, if the signal occurs` within 100 yards of either edge of the 40G-yard range pulse, an additional flow of current will occur in the other diode that is, the second or third half sine wave in the wave 355 in Fig. 16-L will be larger (depending on whether the selected echo 309 is near the left or right edge of the pulse 308) while the current in the rst half of the diode will remain unchanged.

The voltages inthe cathode circuits of the tube V6 are integrated by the condensers connected to them and produce positive signal voltages at the grid of the push-pull direct coupled amplifier 33B. If no signal is present within the 40G-yard range interval spanned by the pulse 308, these voltages are equaland the amplified voltages applied to the tube V1 are equal. Tube V1 com` prises a double triode, the control grids of which receive the output signals of the direct coupled amplifier 33D through resistors 33| and 332. The cathodes are connected together and through a resistor 333 to ground. The control grids are connected to ground through condensers 331i and 335 respectively. The left anode is connected to the positive terminal of the source 245 through resistor 336 while the right anode is connected to the positive terminal of this source through resistors 331. and 33B. The coil oi the A T. I. C. relay 62 is connected across the resistor 331, the relay alsoI having an armature 339 and a conta-ct 34|). Under this condition the plate current of the tube V1 is made insufficient to operate relay 62. If the signal is within plus or minus 100 yards.- of thecenter of the pulse 338, one grid of the tube V1, say for example, the left grid, `is driven in the negative direction. Since the cathode-s of the tube Vl have a common resistor 333, this reduction of the current in the left cathode will reduce the `bias and increase the plate current inthe right section oi the tube V1. If the video signal from the signal selector 22D is of suflicient magnitude, this increase in plate current will cause the relay 62 to operate. A signal occurring Within 100 yards of either edge of the pulse 308- shown in Fig. 16-G causes an increase in the current through the left half of the tube Vl and a decrease in the current through the right half. Under this condition the plate current that 'ows through the relay 333` is insuii`lcient to cause'it. to operate.

The operation of the relay 62 causes the operation of relays 5'4 and 65. The operation ofthe rel lay 65.- causes the armature 342 to make contact 

